Jeremy Corbyn and Sadiq Khan will join members of the Jewish community in a march commemorating the anti-fascist battle of Cable Street 80 years ago.
Campaigners will take to the streets of London's East End on Sunday to protest against the rising number of racist and anti-Semitic offences in the city.
The number of hate crimes reported in the capital has risen to 50 a day since the EU referendum in June.
There were 4,986 racially motivated offences in London between June 24 and September 30, compared with 3,620 in the 99 days before the Brexit vote, according to figures from the mayor of London's office.
Mr Khan is expected to say at the event: "London today is more diverse than ever and, on the whole, is a shining example of how people from different backgrounds can live side-by-side. I am proud that in London, we don't simply tolerate each other, but respect and celebrate our backgrounds and differences.
"When we see things that are divisive, go against our British values and are just fundamentally wrong, we must call them out. Anyone who threatens our values - whether fascists, extremists or political forces seeking to divide our society today - you will fail.
"By working together, playing together, studying together, having a shared set of common values and laws that are clear and progressive, our communities can unite and stand together against anyone who seeks to divide us."
The mayor is due to speak at a Jewish Community event in Shoreditch Town Hall.
Tens of thousands of anti-fascist protesters clashed with police in Cable Street on Sunday October 4 1936.
They were campaigning against a march by members of the British Union of Fascists, led by Oswald Mosley.